A composition is known that is self-curing under atmospheric conditions after application on a surface. This mixture comprises a liquid polysulfide mercaptan group-terminated polymer (molecular weight 4000, cross-linking 0.5%, mercaptan groups 0.53 mol/kg), a peroxide of an alkali or alkaline earth metal as a watersoluble curing agent, an alkaline or alkaline earth oxide, hydroxide or salt of a weak acid as a water-soluble activating agent as well as other modifying additives and fillers. See U.S. Pat. No. 3,225,017.
Drawbacks of the composition: the curing agents are highly inflammable and unstable peroxide compounds; the curing rate is unpredictable since it depends directly on the moisture present in the air; the presence of mercapto groups in the polymer imparts an unpleasant smell to the composition and may provoke cyclic polycondensation or preliminary curing during storage.
A curable composition is known to those skilled in the art in U.S. Pat. No. 3,951,898 as well as a method of curing this composition, which consists in mixing at ambiant temperature: (a) polysulfide polymeric latex ZW-679 (10-20 pbw) which represents an aqueous dispersion containing about 60% of a solid phase linear polymer, with a molecular weight of 100 000 and the formula. EQU --(R--S.sub.x).sub.n --,
wherein S.sub.x is a polysulfide linkage, R is oxahydrocarbon, n is about 500; (b) a polyurethane prepolymer (15-30% of the total amount of the mixture and 4-10% isocyanate groups; (c) fillers; (d) plasticizers, (e) a latex stabilizer and (f) an antisponging agent.
A disadvantage of this composition is that no real curing (cross-linking) of the polysulfide polymer occurs because the composition contains neither branches along the linear chain nor mercapto groups by means of which curing or cocuring with the polyurethane copolymer would occur. Besides, the composition contains no curing agent. In fact the mixture hardens as a result of hydrolysis of the isocyanate groups of the polyurethane under the effect of the water of the latex dispersion, while the polysulfide polymer densifies by coalescence without cross-linking and, as a result, a series of the properties of the cured product are not good enough.
A polysulfide polymer is known from U.S. Pat. No. 4,608,433 which has a molecular weight of 3000 to 100 000 and the formula ##STR1## wherein R and R' are independent polyvalent organic radicals, Y is a terminating inert monovalent substituent, m is a positive integer n ranges from 2 to 8, and p and l are equal to zero or are positive integers.
The drawback of this polysulfide polymer is that it is used as a plasticizer only.